This is probably generic to most ABS bikes. And the critical post wasn't mine - I can't find the original or I'd have posted this as a followup.
Symptoms, brakes initially firm, then the lever heads for the bars. Give them a real quick poke and the ABS will even work.
Pump them enough times (with the bike parked) and they'll come up firm again. Let the lever out wait a few seconds and useless brakes once again. That's what distinguishes this from master cylinder problems.
So, once I'd rebuilt the master cylinder I decided I had to have a problem in the ABS unit
. And with 120,000k's on the bike but the bike otherwise in great shape - well really ... that's just annoying.
After having it parked for a few weeks I decided to try a desperation move, we have quite a large backyard with about 70m of grass diagonally, (and it's raining gently - just what I need, wet grass PR4's and extreme braking ) get the bike down there without falling off, up to about 30kph, hit the front brake hard - try not to fall off. Turn the bike around - try not to fall off turning it around, repeat. After around a dozen passes the brakes came back. So, crap in the main ABS valve and eventually it came loose again.
Not guaranteeing it'll always work and it took a LOT of ABS action to achieve - more than a shop would normally bother with. On the other hand much less $$$ than a new ABS unit.
I've seen a few posts describing similar problems and I hope this is of some help to other ABS equipped bike owners. Note that the original post that made me think of trying this suggested a few other maintenance tips for ABS equipped bikes, which I'll be following in future.
And, really impressed with the ABS, I actually stopped far better than I would have dreamed was possible on wet grass on tires that have near no grip on this surface.